I use autocomplete for character names and locations.

But isn’t there a different feature that autosuggests how any general word might be finished as you are typing?

Does Scrivener have this feature? Or does Mac have this? Does anyone know how to invoke it?

Experiment with the options at Scrivener Preferences > Corrections > Auto-Completion.

For general usage, unticking the In script mode only option might meet your needs.

The other place to look is in the Project ▸ Project Settings… pane, under Auto-Complete List. Here you will find previously typed character names and locations, and this is where you could correct mistakes.

If you are writing scripts, do note that to make a completion available everywhere you need to set its scope to “All”. This setting can be ignored outside of scriptwriting.

You might also be thinking of the Mac option
Apple → System Preferences → Keyboard → Text → “Correct spelling automatically.”